[otbn] remove unused subroutines for rsa verify
[otbn] remove unused subroutines for rsa verify

This removes some subroutines for the verify only
RSA fork. The routines are not needed anymore, since
they are either related to full-sized modexp
for signature generation or since they have been
inlined for the variable time modifications.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan